Composite materials consisting of a thermoplastic matrix and reinforcing fibers, preferably glass fibers, are of increasing interest in many branches of industry, in particular in the automobile industry. Materials which are processed by forming sheet-shaped semifinished products, in particular by flowmolding, are of particular interest in this regard. The best known materials of this kind are glass mat-reinforced thermoplastics (GMT).
For the production of composite materials of this kind, double-belt presses are preferably used for reasons of economy. DE-A-29 48 235 describes a development of the process which is particularly economical from a thermal point of view and in which the thermoplastic is introduced in a molten form. However, the formation of the sheet edge is unsatisfactory in this process. The undefined flowing of the material at the edge results in a relatively wide edge zone of the sheet in which the glass content and sheet thickness fluctuate greatly, so that considerable wastage results.
DE-A-33 47 877 describes a double-belt press for producing chipboards, fiberboards, plywood, electrical laminates or the like in which press a wire in a groove at the outer edges of the belts is carried along at the same speed as the material being pressed. When high pressures are used, such as in the production of GMT from thermoplastics, this lateral sealing, however, does not meet the aim, since the wire is forced out laterally by the material being pressed. In addition, the wire does not provide any sealing in the feed region of the belts.
DE-A-29 23 036 describes a double-belt press for producing rubber belts reinforced with steel cable inserts which are suitable as conveyor belts. Lateral sealing is provided by co-running rubber belts which are supported by bolts fastened on the press belts. Such a double-belt press would not be suitable for producing fiber composite materials from a thermoplastic matrix and glass fiber mats, since simple rubber belts would not stand up to the thermal and mechanical stresses occurring there.
According to EP-A-Q 484 735, the problem of lateral sealing is to be solved by laterally co-running belts consisting of rubber containing reinforcing inserts. In particular, metal link belts are proposed which are coated with a silicone rubber. In the event of an increase in pressure, the belts yield outwardly, the pressure which can be exerted on the material being pressed, however, being limited by the force which can be transmitted to the belts by means of friction. This process is therefore not suitable for pressing at high pressures, which is desirable in the interest of product quality.
Since the belt yields outwardly even in the case of pressure fluctuations within the tolerance limit, a non-uniform edge results which occurs as waste.